A modular converter with a multilevel topology employing a plurality of high-power semiconductor modules may be well suited for many applications, as an increase in voltage may be achieved by increasing a number of levels. Also redundancy may be achieved by employing a plurality of such semiconductor modules. However, when a semiconductor module of a multilevel converter system is faulty, it may be desired to short the module's circuit in order to keep the system functioning in spite of the fault. The multilevel converter system then runs with one less module, but if the remaining modules are still operating within their safe limits, power conversion may continue almost without any noticeable change in output.
While so-called bipolar press-pack devices employed in modular multilevel converter systems, such as e.g. phase-controlled thyristors, diodes, GTOs, IGBTs or IGCTs, can be made to short-circuit upon a failure, not all conceivable failure events can be assumed to lead to a short-circuit reliably; the devices may be blocking but not controllable, i.e. they may not be turned on or off. An exemplary failure may be that control of a switch of a semiconductor module is lost for some reason, e.g. due to a faulty gate-unit, a broken fibre or any other fault. If the semiconductor module fails to close, a capacitor of the module may be over-charged or a highly increased voltage may eventually cause a breakdown of a part of the circuit, which in turn may result in a very energetic surge-current due to a release of the energy stored in the capacitor. A mechanical bypass switch, such as for example a magnetic or pyrotechnic switch, with a separate control may be a solution for ultimately bypassing a semiconductor module and putting it out of operation. Such a mechanical bypass switch may take up valuable module space, may be expensive and may involve regular service to ensure proper functioning.